Anaphylaxis

• Background    

  

  Pathophysiology

  • Severe type 1 hypersensitivity reaction i.e. IgE-mediated mast cell activation.
  • Vasodilation leads to distributive shock and capillary leak leads to angio-oedema.

  Causes

  • Food (50%): peanuts and tree nuts, eggs, fish and shellfish, strawberries.
  • Drugs: penicillin, NSAIDs, opioids, anaesthetic agents (especially muscle relaxants), contrast.
  • Bee or wasp stings.
  • Latex

  Nonallergic anaphylaxis

  • Non-IgE mediated mast cell activation.
  • Aka nonimmunologic anaphylaxis. Formerly known as anaphylactoid reaction.
  • Similar presentation and management to allergic anaphylaxis.
  • Causes: contrast, vancomycin, opioids, anaesthetic agents.

  Epidemiology

  • 1/1000 lifetime risk.
  • 1/10,000 annual incidence.
  • 20 UK deaths annually.

• Signs and symptoms

  Airway, breathing, or circulation problems:
  • Circulation: ↓BP and ↑HR, loss of consciousness.
  • Airway: SOB, cyanosis, stridor due to laryngeal oedema, wheezing due to bronchospasm.
  Other allergic features are common:
  • Angio-oedema: eyes, lips, hands, feet.
  • Skin: itch, sweating, erythema, urticaria.
  • GI: diarrhoea, vomiting, abdo pain.
  Time course:
  • Onset usually seconds or minutes post-exposure, though can be hours, and lasts minutes to hours.
  • 5-20% have a biphasic reaction, with recurrence of symptoms in the following 12 hours (though can be up to 72 hours).

• Diagnosis

  Clinical diagnosis, involving rapid onset (minutes-hours) plus one of 3 criteria:
  1. Skin/mucosa symptoms + cardiac/respiratory compromise.
  2. Exposure to likely allergen for patient + ≥2 organ systems affected (skin/mucosa, GI, cardiac, respiratory).
  3. Exposure to known allergen for patient + ↓BP.
  Serum tryptase:
  • Marker of mast-cell granulation that peaks after 1 hr and remains elevated for 6 hrs. Measure ASAP and at 1-2 hrs.
  • 95% sensitive and specific.
  • Does not aid acute diagnosis but useful in follow up clinic to help support/refute diagnosis.

• Management

  Initial

  1. Intubate if airway compromised and give oxygen as needed.
  2. Remove cause if still present e.g. insect sting.
  3. Adrenaline 500 micrograms IM (0.5 ml 1:1000) in middle, anterolateral thigh:
  • 1:1000 means 1 g per 1000 ml i.e. 1 mg per 1 ml.
  • Repeat every 5 minutes until hemodynamic improvement. Switch to IV infusion if refractory.
  • Children: 150 micrograms (0.15 ml) <6 years, 300 micrograms (0.3 ml) 6-12 years.
  • Mechanisms: reduces capillary leak and vasodilation (α1-receptors), is a +ve inotrope and chronotrope (β1-receptors), relaxes airway smooth muscle (β2-receptors), and by reducing mast cell degranulation it acts on the whole process, including urticaria, itch, and angio-oedema.
  4. Get IV access and give fluids: 1 L crystalloid.
  5. Further drugs:
  • Chlorphenamine 10 mg IV to reduce itch and hives.
  • Hydrocortisone 200 mg IV may reduce prolonged or biphasic reactions.
  • Neither actually treat the anaphylaxis and the evidence for them isn't great.
  6. If there is continued wheeze, treat as you would asthma:
  • Salbutamol, ipratropium, or magnesium sulphate.

  Resolution

  Observe 6-12 hrs.
  On discharge:
  • Warn about possible biphasic reaction.
  • Advise to avoid any potential triggers
  • Give 2 pre-loaded 300 microgram adrenaline autoinjectors as interim until clinic.
  Refer to allergy clinic to identify allergen and provide long-term care.